1. Field of the Invention
This invention relates to the field of measuring technology, and more particularly to the field of measuring the thickness of clad material such as clad metal.
2. Description of the Related Arts
Generally speaking, so called "clad materials" in steel technology means clad metals wherein a base metal made of low carbon steel or low alloyed steel, hereafter mother metal, is clad on one or both sides by an anti-corrosive metal such as a stainless steel or a nickel base alloy, hereafter clad metal.
Generally the clad surface is metallurgically connected. The thickness of the clad metal, hereafter clad thickness, the thickness of the mother metal, and the total thickness or the thickness of the mother metal plus the clad thickness, hereafter the total thickness, should be accurately measured.
As for the method of measuring these thicknesses, a measuring technique using an optical method is proposed wherein the end sections of a clad product are cut, polished and observed by a microscope.
Japanese Utility Model laid open Nos. 73205/1987 and 143205/1982 propose methods of measuring the thickness wherein the total thickness is measured by an ultrasonic thickness meter and the clad thickness is measured by an eddy current thickness meter, or a film thickness meter. As for the former method, for instance, when the ends of a clad material are cut, polished, and observed by a microscope, a yield loss is generated due to the cutting, and a considerable amount of time is required for the measuring.
Moreover, it is impossible to measure the thicknesses of the clad material with respect to its whole length. As for the latter method, when this measuring method is applied to a clad tube wherein a clad part is inserted into the inside of a mother tube, the total thickness is measured by the ultrasonic thickness meter from the outside of the clad tube, and the clad thickness is measured by the eddy current thickness meter utilizing the principle of a film thickness meter from the inside of the clad tube. Due to the principle of measurement, the clad metal is restricted to a nonmagnetic one. To carry out the measurement, a sensor of the eddy current thickness meter should be inserted into the clad tube which requires a considerable facility with a considerable mechanism. In the measuring operation, the insertion and taking out of the sensor requires a considerable amount of time.
Especially when a long clad tube is measured by this device, the mechanism for the inserting and the positioning of the sensor requires a considerable facility and the positioning is so difficult that the discrepancy of the measured points between the sensor of the eddy current thickness meter and that of the total thickness meter needs to be corrected.